One technology for after-treatment of engine exhaust utilizes SCR to enable certain chemical reactions to occur between NOx in the exhaust and ammonia (NH3) introduced into an engine exhaust system upstream of an SCR catalyst to entrain with exhaust flowing toward the SCR catalyst on surfaces of which ammonia is stored. Those reactions convert NOx into Nitrogen (N2) and water (H2O), two constituents found in abundance in earth's atmosphere. NOx and NH3 are the only reactants in certain of those reactions while Oxygen (O2), which may be present in the exhaust, is a third reactant in other reactions.
Ammonia is introduced in sufficient quantity to maintain a presence of ammonia on surfaces of the SCR catalyst where the chemical reactions which reduce NOx take place.